There have been widely known electrophotographic photoreceptors with a light-sensitive layer comprising an inorganic photoconductor such as selenium, zinc oxide, cadmium sulfide or silicon as a main component. But these materials are not entirely satisfactory in properties such as thermal resistance and durability, besides problems in manufacturing and handling
On the contrary, a photoreceptor having a light-sensitive layer comprising an organic photoconductor has many advantages including easiness in manufacture, inexpensiveness, simplicity in handling, and better thermal stability as compared with a selenium photoreceptor. Poly-N-vinylcarbazole is best known as such an organic photoconductive compound, and there has been practically used a photoreceptor having a light-sensitive layer which mainly comprises of poly-N-vinylcarbazole and a charge-transfer complex formed from a Lewis acid such as 2,4,7-trinitro-9-fluorenone and polyvinylcarbazole.
Also, there is known a photoreceptor having a function-separated photoconductive layer of laminated or single-layered type, in which functions of carrier-generation and carrier-transfer are separately allotted to different materials. For example, there has come to be practically used a photoreceptor comprising a carrier-generation layer composed of a thin amorphous selenium layer and a carrier-transfer layer mainly made up from poly-N-vinylcarbazole.
However, poly-N-vinylcarbazole is less flexible, and its film is liable to crack and peel off, thereby a photoreceptor formed from this material is poor in durability. Addition of plasticizer to correct this defect tends to increase residual potential while an electrophotographic process is performed, and repetition of the process causes the residual potential to accumulate and accelerates the generation of fog which substantially impairs copied images.
Further, since low molecular weight organic photoconductive compounds cannot form a film in general, they are used in combination with an appropriate binder. While it is convenient that physical properties and light-sensitive characteristics of the film can be controlled to same extent by selecting a type and addition amount of a binder, the number of organic photoconductive compounds having a high compatibility with a binder is not many. Actually, only few binders can be used in forming a light-sensitive layer of an electrophotographic photoreceptor.
For example, 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole described in U.S. Pat. No. 3,189,447 is less compatible with polyester and polycarbonate both of which are commonly used as a component of light-sensitive layer of an electrophotographic photoreceptor. When a light-sensitive layer is formed by incorporating this material at a percentage necessary to impart electrophotographic properties, crystals of oxadiazole come to form at a temperature of 50.degree. C. or above, deteriorating electrophotographic properties such as charge-holding power and sensitivity.
Diaryl alkane derivatives described in U.S. Pat. No. 3,820,989 have less problems with respect to compatibility with a binder. But their light stability is poor; therefore, when they are used in a light-sensitive layer of a photoreceptor for repetitive transfer type electrophotography in which a cycle of electrification and exposure is repeated, the sensitivity of said light-sensitive layer gradually deteriorates.
Different types of phenothiazine derivatives are described in U.S. Pat. No. 3,274,000 and Japanese Patent Examined Publication No. 36428/1972, respectively. But each of them has shortcomings of low sensitivity and low stability in a repetitive use.
Further, stilbene compounds described in Japanese Patent Publication Open to Public Inspection Nos. 65440/1983 and 190953/1983 have a good charge-holding power and sensitivity, but their poor durability is unfit for repetitive uses.
As mentioned above, there has not been found a carrier-transfer material exhibiting satisfactory properties when practically used in an electrophotographic photoreceptor.